segunda-feira, 9 de março de 2015

El Niño arrives later and weaker than expected

The El Niño weather pattern that scientists have awaited for nearly a
year has finally arrived. But forecasters say that its signature
warming of the equatorial Pacific Ocean will not have a major impact on
global weather.
“This Niño is weak in
strength, and it’s also quite late,” says Mike Halpert, deputy director
of the US National Oceanic and Atmospheric Administration’s (NOAA)
Climate Prediction Center in College Park, Maryland.
El
Niños usually develop in the latter part of the year and peak from
December to April. They are often associated with widespread changes in
precipitation patterns, as well as warmer global temperatures. But
NOAA’s latest forecast, issued on 5 March, suggests that the weakness
and timing of the current event will minimize its impact. Some effects
associated with El Niño could surface in the coming months, including
additional precipitation along the US Gulf Coast, but Halpert says this
one is probably “too little, too late and too weak” to boost rainfall
along the west coast and provide relief for drought-stricken California.

NOAA

Warmer than normal surface temperatures (red) in the equatorial Pacific Ocean indicate the arrival of an El Niño.

The announcement comes a year after
forecasters first predicted that a major El Niño could be in the works.
At the time, NOAA predicted a 50% chance that an El Niño could develop
in the latter half of 2014. The agency also said the wind patterns that
were driving water east across the Pacific were similar to those that
occurred in the months leading up to the epic El Niño of 1997, which
caught scientists by surprise and contributed to flooding, droughts and
fires across multiple continents.
In the end,
last year's forecasts came up short, in part because the winds that were
driving the system petered out. Researchers, who have been working to
improve their forecasting models since 1997, are trying to figure out precisely what happened last year and why their models failed to capture it.
“Why
did these two years that looked so similar early on turn out so
differently?” asks Gabriel Vecchi, a climate modeller at NOAA’s
Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey.

Vecchi
calls the 2014–15 season “a great case-study” and says such unusual
events can be useful for scientists. “They force us to challenge our
assumptions, to revisit our rules of thumb and to seek fuller
explanations and more robust models,” he says.

Forecasters are
trying to increase the amount of lead time they can give the world in
the event of another major El Niño, but the challenge is that the
weather patterns in the equatorial Pacific are fickle in the first half
of the year; even significant trends can stall or even reverse course.
By July,
the models are more reliable and forecasters are better able to predict
what kind of patterns might develop in the latter half of the year. For
now, NOAA is predicting a 50–60% chance that El Niño conditions could
continue into the Northern Hemisphere summer.